1. Field of the Invention
The present invention relates to a transmission power control scheme for a radio communication used in a mobile communication system.
2. Description of the Background Art
In a radio communication, a transmission power is usually controlled in order to suppress the transmission power. By means of this transmission power control, it is possible to realize advantageous effects such as a saving of a power consumption and a reduction of an interference to the other radio channels. In particular, such a transmission power control is indispensable in the CDMA (Code Division Multiple Access) scheme, as a reduction of an amount of interference with the other radio channels can directly lead to an increase of a subscriber capacity in the CDMA scheme.
Conventionally, there has been a proposition for a transmission power control in the CDMA scheme in which a transmission power of a mobile station is controlled such that a reception CIR (Carrier to Interference Ratio) at a base station becomes equal to a target CIR, while a transmission power of a base station is controlled such that a reception CIR at a mobile station becomes equal to a target CIR.
FIG. 1 shows an exemplary mobile communication system having a mobile station 101 and a base station 103, in which a conventional mobile station transmission power control scheme as indicated in FIG. 2 is carried out. In this conventional scheme, when the reception CIR at the base station 103 becomes lower than the target CIR, the base station 103 sends a transmission power control signal with a value "1" to the mobile station 101 in order to raise the transmission power at the mobile station 101. When this transmission power control signal with a value "1" is received, the mobile station 101 raises its transmission power for a prescribed amount such as 1 dB for example. On the contrary, when the reception CIR at the base station 103 becomes higher than the target CIR, the base station 103 sends the transmission power control signal with a value "0" to the mobile station 101 in order to lower the transmission power at the mobile station 101. When this transmission power control signal with a value "0" is received, the mobile station 101 lowers its transmission power for a prescribed amount such as 1 dB for example.
In practice, even when the transmission power is controlled to make the reception CIR equal to the target CIR, due to the measurement error or the control delay, it is generally impossible to make the reception CIR completely equal to the target CIR, and the reception CIR varies in time, so that there is a control error.
Here, an amount of variation of the reception CIR depends on a fading pitch and a number of reception paths. In the radio transmission, a plurality of delay waves at mutually different timings are generated via different paths. In the CDMA scheme, a wide bandwidth transmission using spread codes is carried out, so that it is possible to receive the aforementioned plurality of delay waves separately, and the reception characteristic can be improved by composing these separately received delay waves together. In general, a number of paths is greater for an area with more reflection objects such as an urban area.
Namely, when the fading pitch is small, the transmission power control cannot keep up with the fading variation due to the control delay, and an amount of variation of the CIR becomes large. On the contrary, when the fading pitch is large, the transmission power control can keep up with the fading variation, so that an amount of variation of the CIR becomes small. Also, when a number of reception paths is greater, it is possible to make an influence of the fading variation smaller by means of the path composition, so that an amount of variation of the CIR becomes smaller.
Now, in the conventional transmission power control scheme as described above, the target CIR is set to be a fixed value as shown in FIG. 3, with a sufficient margin above the required CIR such that the required communication quality can be satisfied even in a case of the largest variation of the reception CIR such as a case in which the mobile station is moving fast and there are only few paths available.
However, with this fixed setting of the target CIR, there is going to be a more than necessary amount of margin in a case an amount of variation of the reception CIR is small, so that the transmission power of the mobile station and the base station becomes higher than what is really necessary in such a case. Consequently, the conventional transmission power control involves the wasteful consumption of the transmission power and the unnecessary increase of the amount of interference with the other channels which in turn causes the unnecessary lowering of the subscriber capacity.
Moreover, according to the conventional transmission power control scheme as described above, it is necessary to set up the fixed target CIR by measuring or estimating an amount of variation of the reception CIR in advance at a time of system designing, which requires an extra amount of works at a time of system designing.